Various methods are known as an image forming method based on the electrophotographic method. In general, by using a photoconductive substance, an electrostatic latent image is formed on an electrostatic image carrying member (hereinafter, also referred to as “photosensitive member”) by using various techniques. Then, by developing with a toner, the electrostatic latent image is converted into a visible image, and the visible image formed with the toner is transferred onto a recording medium such as paper, according to need, and thereafter fixed as a toner image on the recording medium by heat or pressure to yield a copy. Examples of the image forming machines for forming such a copy include printers and copying machines.
In these years, as printer apparatuses, LED laser beam printers become the mainstream in the market, and the resolution has become higher as seen in transition from conventional resolutions of 300 dpi and 400 dpi to higher resolution of 600 dpi and 1200 dpi. Accordingly, the development method has been required to be higher in definition. Copying machines have also become increasingly higher in performance through digitalization, and have been required to involve development methods higher in resolution and definition similarly to printers.
Usually, the toners used in these printers and copying machines are microparticles including as main constituent materials a binder resin and colorants such as dyes, pigments, carbon black and magnetic materials, and toners of approximately 5 to 30 μm in particle size are used.
A toner is generally produced by a so-called pulverizing method in which in a thermoplastic resin as a binder resin, the above-described colorants and, according to need, a charge controlling agent and a wax are melt-mixed so as to be uniformly dispersed, and thereafter the thus obtained resin composition is finely pulverized and classified to obtain desired particle sizes. In this method for producing a toner, the prerequisites to be satisfied by the constituent materials include, for example, a point that the resin composition should be sufficiently brittle and be able to be finely pulverized with an economically feasible production apparatus. However, when the brittleness of the resin composition is made higher, there occurs a problem that the particle size range of the particles obtained by fine pulverization tends to be broadened. Additionally, caused is a problem that even after completion of the toner production, the toner tends to be further finely pulverized while being used in the development unit, and the colorants are exposed to the fracture surface of the toner particles to cause the degradation of the developability.
On the other hand, for the purpose of overcoming the problems of the toner based on such a pulverizing method, a method for producing a polymerized toner by means of a suspension polymerization method has been proposed. The suspension polymerization method is a method in which a polymerizable monomer composition, including a polymerizable monomer and the substances required to be encapsulated in the toner particle such as a colorant and, according to need, a multifunctional monomer, a chain transfer agent, a charge controlling agent and a wax dissolved or dispersed in the polymerizable monomer, is suspended in an aqueous medium containing a dispersion stabilizer as well as a polymerization initiator, and the suspension liquid is subjected to polymerization by means of a method such as heating to yield a toner as particles having a desired particle size. According to this method, no pulverization step is included, and hence the resin material is not required to have brittleness and even a soft resin material can be used. Additionally, the colorant is hardly exposed to the surface of the toner particles, and hence a toner particle uniform in frictional chargeability and excellent in durability can be obtained. Further, the classification step can also be omitted, and hence cost reduction effects such as energy saving, production time reduction and yield improvement become significant.
However, carbon black and some dyes and pigments used as the colorants include substances tending to inhibit the polymerization reaction. Additionally, in the polymerized toner produced by the suspension polymerization method or the resin produced by the suspension polymerization method, depending on the type of the used polymerization initiator, the unreacted polymerizable monomer may remain in the toner particles or the resin particles. When the amount of the remaining polymerizable monomer is too large, the charge amounts of the individual toner particles become nonuniform to facilitate fogging, and the contamination of the toner carrying member and the filming to the photosensitive member tend to be caused, and hence there is caused a problem that the image quality is degraded.
Additionally, the utilization efficiency of the polymerization initiator in the suspension polymerization method is not necessarily sufficient, and a part of the polymerization initiator is not involved in the polymerization reaction and may remain in the toner particles or the resin as decomposition product residues. The decomposition product residues are produced from the compounds in the reaction system other than the polymerization initiator through the hydrogen abstraction by the free radicals (radicals) produced by the decomposition of the polymerization initiator, and from the mutual disproportion and mutual recombination of the radicals; the decomposition product residues mainly include compounds such as alcohols, carboxylic acids and hydrocarbons. Among the decomposition products, low boiling point products can be distilled off by conducting, after polymerization, operations such as heating and pressure reduction, and water-soluble products can be eluted into aqueous media; however, relatively high molecular weight, high boiling point and slightly soluble compounds are hardly removable and consequently remain in the toner particles.
Such decomposition product residues also offer causes for the degradation of the charge stability and the degradation of the image quality in long term use, and offer a cause for the so-called high-temperature offset in which the molten toner tends to adhere to the heating roller at the time of development and the thus adhered toner contaminates a sheet to be fixed. Additionally, a large amount of production of such decomposition products gives rise to the decrease of the utilization efficiency of the polymerization initiator, and such decrease offers a cause for the increase of the amount of the unreacted polymerizable monomer.
There have hitherto been intensively carried out investigations for preventing the remaining presence, in the toner particles, of the decomposition product residues derived from the unreacted polymerizable monomer and the polymerization initiator; examples of such proposals include the following various methods.
For example, there has been proposed a method for producing a toner resin in which the content of the decomposition product residues derived from the polymerization initiator is reduced by using as a polymerization initiator a peroxide having a specific structure and a 10-hour half-life temperature of 120° C. or lower (see Japanese Patent Application Laid-Open No. S61-114245).
Additionally, there has been proposed a method for obtaining a toner resin in which the remaining presence of the unreacted monomer (polymerizable monomer) is suppressed by conducting the polymerization in the simultaneous presence of a polymerization initiator having a specific structure other than the structure of the above-described polymerization initiator and a 10-hour half-life temperature of 70° C. or higher and an additional polymerization initiator (see Japanese Patent Application Laid-Open No. H07-181731).
Further, for production of a polymerized toner for use in a nonmagnetic one-component developer, there has been proposed a method for producing a polymerized toner in which the amount of the decomposition products derived from the polymerization initiator and the amount of the remaining monomer (polymerizable monomer) are suppressed by using as a polymerization initiator a nonaromatic organic peroxide having a molecular weight of 250 or less and a 10-hour half-life temperature of from 60 to 85° C. and by conducting suspension polymerization at a polymerization temperature of from 75 to 100° C. (see Japanese Patent No. 3336862).
Of the above-described conventional art examples, the method disclosed in Japanese Patent Application Laid-Open No. S61-114245 uses an aliphatic organic peroxide as a polymerization initiator, examples of such a peroxide including, in particular, organic peroxides limited in the number of the carbon atoms of the aliphatic hydrocarbon group among common peroxycarbonate organic peroxides, monocarbonate organic peroxides, diacyl organic peroxides, dicarbonate organic peroxides. According to this method, the decomposition products derived from the polymerization initiator have relatively low molecular weights. Consequently, when a binder resin for use in toner is produced by using such a polymerization initiator on the basis of a solution polymerization method, the decomposition product residues are evaporated by high-temperature heating in the solvent removal step after polymerization and in a melt-kneading step in the toner preparation, and hence, as disclosed, the remaining presence of the decomposition product residues in the toner particles can be suppressed. However, when such a polymerization initiator is applied to the production of a suspension-polymerized toner, the above-described steps each involving a high temperature heating treatment are not included, and hence it is difficult to suppress the remaining presence of the decomposition product residues in the toner particles. Additionally, it has also been found difficult to suppress the polymerization inhibition due to some colorants.
The above-described method disclosed in Japanese Patent Application Laid-Open No. H07-181731 uses a polymerization initiator that produces radicals hardly causing hydrogen abstraction reaction in a step of producing a binder resin for use in toner. According to this method, the radicals can stably persist over a long period of time, and hence, as disclosed, the utilization efficiency of the monomer is improved, and the remaining presence of the unreacted monomer can be suppressed. However, this polymerization initiator has a high 10-hour half-life temperature and is not necessarily suitable as a polymerization initiator for use in the production of a suspension-polymerized toner. Additionally, this polymerization initiator produces other radicals as well as the radicals hardly causing hydrogen abstraction reaction, and further, another polymerization initiator is needed to be simultaneously used, and the effect of reducing the produced amounts of the decomposition product residues has been found small.
The above-described method disclosed in Japanese Patent No. 3336862 specifies, in the production of a polymerized toner based on a suspension polymerization method, the molecular weight and the 10-hour half-life temperature of the used polymerization initiator, and intends to thereby suppress the remaining presence of the decomposition product residues and the unreacted monomer. However, the physical properties of the decomposition products are not uniquely determined only by the molecular weight of the polymerization initiator, but are controlled by the molecular weights and the molecular structures of the decomposition products themselves. Also, the amount of the unreacted monomer is not simply determined only by the 10-hour half-life temperature of the polymerization initiator, but depends to a great degree on the balance between the 10-hour half-life temperature and the polymerization temperature. This method intends to suppress the remaining presence of the decomposition product residues in the toner particles, but not to suppress the production itself of the decomposition products. According to the investigation by the present inventors, this method still has room to be improved with respect to the remaining presence of the decomposition product residues and the unreacted monomer.
As described above, as affairs stand now, with respect to the polymerized toner produced by a suspension polymerization method, there have never been developed production methods that can solve various problems caused by the remaining presence of the unreacted polymerizable monomer and the decomposition product residues in the toner particles.
An object of the present invention is to provide a method for producing a toner and a method for producing a binder resin for use in toner that have solved the above-described conventional problems.
In other words, the object of the present invention is to improve the utilization efficiency of the polymerization initiator used in the production of the toner or the binder resin for use in toner.
Additionally, another object of the present invention is to provide a production method that can reduce the effects of the polymerization inhibiting substances.
Additionally, another object of the present invention is to provide a production method that can suppress the remaining presence, in the toner particles, of the decomposition product residues derived from the unreacted polymerizable monomer and the polymerization initiator.
Additionally, another object of the present invention is to provide, by using the above-described production method, a toner or a binder resin for use in toner that is excellent in triboelectric charging stability and can yield stable images over a long term.